International Telecommunication Union
ITU-T
Introduction to the
Path Computation Element
Adrian Farrel
Old Dog Consulting
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
Agenda
ITU-T
o Domains, Path Computation, and TE
o
o
o
o
Abstraction
The Path Computation Element
Basic Architectural Possibilities
PCE-Based Path Computation Examples
Applying PCE to The ASON Architecture
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
2
Path Computation
ITU-T
o
The selection of the route through the
network taken by a connection
1. Full precomputation
2. On-demand full computation
3. Segment-by-segment computation
4. Hop-by-hop computation
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
3
Domains
ITU-T
o
o
“Any collection of network elements
within a common sphere of address
management or path computational
responsibility.”
Classic examples…
1. IGP Areas
2. Autonomous Systems
o
ASON examples…
1. Sub-networks
2. Routing levels
3. Layers
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
4
Interdomain Routing Issues
ITU-T
o
o
o
o
o
o
TE visibility is (completely) limited
Which domain contains the destination?
Which domain to go to next?
Which connection to the next domain?
How to guarantee path diversity for
protection
Questions apply to:
1. Reachability
2. Acceptable paths
3. Optimal paths
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
5
TE Abstraction – A Rejected Option
ITU-T
Virtual Link
o “You can reach this
destination across this
domain with these
characteristics”
o BGP-TE model
o Requires large amount of
information
o Needs frequent updates
Virtual Node
o Hierarchical abstraction
o Presents subnetwork as a
virtual switch
o Can be very deceptive
• No easy way to advertise
“limited cross-connect
capabilities”
Both rely on crankback signaling and high CPU aggregation
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
6
Path Computation Element (PCE)
ITU-T
o
A new functional component
1.
2.
3.
4.
o
Performs path computation
Preserves confidentiality
Avoids abstraction/aggregation issues
Off-loads computational complexity
THE IETF’s PCE working group defines…
• Path Computation Element: an entity
(component, application or network node)
that is capable of computing a network
path or route based on a network graph
and applying computational constraints.
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
7
PCE as an NMS Tool
ITU-T
o PCE can be a tool used by the NMS
• May be built into NMS or separate
o Traffic Engineering Database (TED)
• Fed by configuration, inventory, routing
protocols
NMS
Service Request
Request
Response
PCE
TED
Config.
IGP
Service
Request
Signalling Protocol
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
8
PCE in the Connection Controller
ITU-T
o PCE can be collocated with the Connection
Controller
• Easy implementation matches existing code
• Request/response is not a formal interface
TED
Service
Request
Config.
IGP
PCE
Request/Response
CC
Signalling Protocol
CC
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
9
The PCE Server
ITU-T
o PCE implemented as a separate server
• Dedicated server or powerful NE
o Requires formal request/response protocol
TED
Config.
IGP
PCE
Request/Response
Service
Request
CC
Signalling Protocol
CC
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
10
Cooperating PCE Servers
ITU-T
o PCEs in different domains may cooperate
• Use formal request/response protocol
• Increase quality of computed path
Config.
IGP
TED
TED
PCE
Request/Response
PCE
Request/Response
Service
Request
CC
Signalling
Protocol
CC
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
CC
11
End-to-end Independent PCEs
ITU-T
o Computation on entry to each domain
o Sub-optimal end-to-end paths
o Paths may be blocked
• Use crankback to recover
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
12
End-to-end : Cooperating PCEs
ITU-T
3. How should I
reach the Egress?
2. Thinks…
“A looks best”
PCE
PCE
4. Thinks…
“D would be
best”
1. I want to reach
the Egress
7. I want to reach
the Egress
5. Route thru’ B
8. Route thru’ Y
6. Route thru’ X and B
A
C
Ingress
X
Y
Egress
B
D
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
13
Diverse Path Cooperating PCEs
ITU-T
2. Thinks…
“Need to exit
through A and B”
4. Thinks…
“(D or E) and F”
3. I want disjoint
paths to the Egress
7. I want to reach
the Egress
Cookie
PCE
PCE
1. I want disjoint
paths to the Egress
5. Route thru’
(A or B) and C
Cookies
8. Route thru’ Z
9. I want to reach
the Egress
Cookie
6. Route thru’ A and XYC
A
D
10. Route
thru’ PQ
Ingress
X
Z
Egress
B
E
Q
Y
C
P
F
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
14
PCE in the ASON Architecture
ITU-T
o PCE is applicable to the ASON Architecture
o One PCE per sub-network in peer routing
• The model is exactly as just demonstrated
o One PCE per level in multi-level routing
• Hierarchical interaction between PCEs
o One PCE per layer in multi-layer routing
• Hierarchical interaction between PCEs
o New work starting…
• G.7715.2 : ASON routing architecture and
requirements for remote path query
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
15
Questions?
ITU-T
o Background reading
• draft-ietf-pce-architecture-04.txt
• Free download from
http://www.ietf.org/internet-drafts/
o Follow-up off-line
• adrian@olddog.co.uk
ITU-T Workshop “NGN and its Transport Networks“
Kobe, 20-21 April 2006
16